In the field of mobile communication system (for example, personal handyphone system: hereinafter PHS) evolving rapidly this few years, a PDMA (Path Division Multiple Access) system that allows mobile terminal apparatuses of a plurality of users to effect path multiple connection to a radio base system by dividing the same time slot of the same frequency spatially has been proposed in order to improve the usage efficiency of radio frequency. In the PDMA system, the signals from the mobile terminal apparatuses of respective users are separated and extracted by the well-known adaptive array processing. The PDMA system is also called the SDMA system (Spatial Division Multiple Access).
FIG. 18 represents the channel arrangement of the various communication systems of frequency division multiple access (FDMA), time division multiple access (TDMA), and spatial division multiple access (SDMA).
First, FDMA, TDMA and SDMA will be described briefly with reference to FIG. 18. FIG. 18(a) corresponds to FDMA. The analog signals of users 1–4 are subjected to frequency-division and transmitted over radio waves of different frequencies f1–f4. The signals of respective users 1–4 are separated by frequency filters.
FIG. 18(b) corresponds to TDMA. Digitized signals of respective users are transmitted over radio waves at different frequencies f1–f4, and time-divided on the basis of the prescribed period of time (time slot). The signals of respective users are separated by means of frequency filters and time-synchronization between a base station and mobile terminal devices of respective users.
The SDMA system has now been proposed to improve the usage efficiency of radio frequency in accordance with the spread of mobile phones. The SDMA system spatially divides one time slot of the same frequency to transmit data of a plurality of users, as shown in FIG. 18(c). In this SDMA, the signals of respective users are separated by means of frequency filters, time synchronization between a base station and mobile terminal devices of respective users, and a mutual interference canceller such as an adaptive array.
The adaptive array processing set forth above is well known in the field of art, and is described in detail in, for example, “Adaptive Signal Processing by Array Antenna” (Kagaku Gijutsu Shuppan), issued Nov. 25, 1998, pp. 35–49, “Chapter 3: MMSE Adaptive Array” by Nobuyoshi Kikuma. The conventional adaptive array processing will be described briefly hereinafter.
FIG. 19 is a schematic block diagram of a configuration of a transmission and reception system 5000 of a conventional base station for SDMA.
In the configuration shown in FIG. 19, four antennas #1–#4 are provided to establish identification between users PS1 and PS2. In a reception operation, the outputs of antennas are provided to an RF circuit 5101 to be amplified by the reception amplifier, and then frequency-converted by a local oscillation signal. The converted signals have the unnecessary frequency signal removed by filters, then subjected to A/D conversion, and applied to a digital signal processor 5102 as digital signals.
Digital signal processor 5102 includes a channel allocation reference calculator 5103, a channel allocating apparatus 5104, and an adaptive array 5100. Channel allocation reference calculator 5103 calculates in advance whether the signals from the two users can be separated by the adaptive array. Based on the calculation result, channel allocation apparatus 5104 provides channel allocation information including user information, selecting frequency and time, to adaptive array 5100. Adaptive array 5100 applies a weighting operation in real time on the signals from the four antennas #1–#4 based on the channel allocation information to separate only the signals of a particular user.
[Configuration of Adaptive Array Antenna]
FIG. 20 is a block diagram showing a configuration of a transmission and reception unit 5100a corresponding to one user in adaptive array 5100. The example of FIG. 20 has n input ports 5020-1 to 5020-n to extract the signal of the desired user from input signals of a plurality of users.
The signals input to respective input ports 5020-1 to 5020-n are applied via switch circuits 5010-1 to 5010-n to a weight vector control unit 5011 and multipliers 5012-1 to 5012-n. 
Weight vector control unit 5011 calculates weight vectors w1i–wni using input signals, a unique word signal corresponding to signals of a particular user prestored in a memory 5014, and the output from an adder 5013. In the present specification, subscript “i” implies that the weight vector is employed for transmission/reception with the i-th user.
Multipliers 5012-1 to 5012-n multiply the input signals from input ports 5020-1 to 5020-n by weight vectors w1i–wni, respectively. The multiplied results are applied to adder 5013. Adder 5013 adds the output signals from multipliers 5012-1 to 5012-n to output the added signals as a reception signal SRX(t). This reception signal SRX(t) is also provided to weight vector control unit 5011.
Transmission and reception unit 5100 a further includes multipliers 5015-1 to 5015-n receiving and multiplying an output signal STX(t) from an adaptive array radio base station by respective weight vectors w1i–wni applied from weight vector control unit 5011. The outputs of multipliers 5015-1 to 5015-n are provided to switch circuits 5010-1 to 5010-n, respectively. Specifically, switch circuits 5010-1 to 5010-n provide the signals applied from input ports 5020-1 to 5020-n to a signal receiver unit 1R in a signal receiving mode, and provide the signal from a signal transmitter unit 1T to input/output ports 5020-1 to 5020-n. 
[Operating Mechanism of Adaptive Array]
The operating mechanism of transmission and reception unit 5100a of FIG. 20 will be described briefly here.
For the sake of simplifying the description, it is assumed that there are four antenna elements, and that two users PS effect communication at the same time. Here, the signals applied to reception unit 1R from respective antennas are represented by the equations set forth below.RX1(t)=h11Srx1(t)+h12Srx2(t)+n1(t)  (1)RX2(t)=h21Srx1(t)+h22Srx2(t)+n2(t)  (2)RX3(t)=h31Srx1(t)+h32Srx2(t)+n3(t)  (3)RX4(t)=h41Srx1(t)+h42Srx2(t)+n4(t)  (4)
Signal RXj (t) represents a reception signal of the j-th (j=1, 2, 3, 4) antenna. Signal Srxi (t) represents a signal transmitted by the i-th (i=1, 2) user. Coefficient hji represents the complex coefficient of a signal from the i-th user received at the j-th antenna, and nj (t) represents noise included in the j-th reception signal.
The above equations (1)–(4) may be represented in vector form as follows:X(t)=H1Srx1(t)+H2Srx2(t)+N(t)  (5)X(t)=[RX1(t), RX2(t), . . . , RXn(t)]T  (6)Hi=[h1i, h2i, . . . , hni]T, (i=1, 2)  (7)N(t)=[n1(t), n2(t), . . . , nn(t)]T  (8)
In equations (6)–(8), [. . . ]T denotes the transposition of [. . . ]. Here, X (t) represents the input signal vector, Hi the reception signal coefficient vector of the i-th user, and N (t) a noise vector.
The adaptive array antenna outputs as a reception signal SRX(t) a synthesized signal obtained by multiplying the input signals from respective antennas by respective weight coefficients w1i–wni, as shown in FIG. 20. Here, The number of antennas n is 4.
Given these preliminaries, the operation of an adaptive array in the case of extracting a signal Srx1 (t) transmitted by the first user, for example, is set forth below.
Output signal y1 (t) of adaptive array 2100 can be represented by the following equations by multiplying input signal vector X(t) by weight vector W1.y1(t)=X(t) W1T  (9)W1 =[w11, w21, w31 , w41]T  (10)
In other words, weight vector W1 is a vector with the weight coefficients wj1(=1, 2, 3, 4) to be multiplied by the j-th input signal RXj (t) as elements.
Substituting input signal vector X (t) represented by equation (5) into y1 (t) represented by equation (9) yields:y1(t)=H1W1TSrx1(t)+H2W1TSrx2(t)+N(t)W1T  (11)
By a well known method, weight vector w1 is sequentially controlled by weight vector control unit 5011 so as to satisfy the following simultaneous equations when adaptive array 5100 operates in an ideal situation.H1W1T=1  (12)H2W1T=0  (13)
If weight vector W1 is perfectly controlled so as to satisfy equations (12) and (13), output signal y1 (t) from adaptive array 2100 is eventually represented by the following equations.y1(t)=Srx1(t)+N1(t)  (14)N1(t)=n1(t)w11+n2(t)w21+n3(t)w31+n4(t)w41  (15)
Specifically, signal Srx1 (t) transmitted from the first of the two users will be obtained for output signal y1 (t).
In FIG. 20, input signal STX (t) for adaptive array 5100 is applied to transmitter unit 1T in adaptive array 2100 to be applied to respective one inputs of multipliers 5015-1, 5015-2, 5015-3, . . . , 5015-n. To the other inputs of these multipliers, weight vectors w1i, w2i, w3i, . . . , wni calculated by weight vector control unit 5011 based on reception signals described above are copied and applied.
The input signals weighted by these multipliers are delivered to corresponding antennas #1, #2, #3, . . . , #n via corresponding switches 5010-1, 5010-2, 5010-3, 5010-n for transmission.
FIG. 21 is a schematic diagram to describe a configuration of signals transferred between a terminal and SDMA base station 5000.
The signals of 1 frame are divided into 8 slots, the 4 slots of the former half directed to, for example, reception, and the 4 slots of the latter half directed to, for example, transmission.
Each slot is formed of 120 symbols. Based on one slot for reception and one slot for transmission as one set, the signals of 1 frame can be allocated to as many as 4 users in the example of FIG. 21.
Identification of users PS1 and PS2 is established as set forth below. A radio wave signal of a mobile phone is transmitted taking a frame form set forth above. The slot signal from a mobile phone is mainly composed of a preamble formed of a signal series known to a radio base station, and data (voice and the like) formed of a signal series unknown to the radio base station.
The preamble signal series includes a signal stream of information to identify whether the current user is the appropriate user to converse for the radio base station. Weight vector control unit 5011 of adaptive array radio base station 1 compares the unique word signal output from memory 5014 with the received signal series to conduct weight vector control (determination of weight coefficient) so as to extract the signal expected to include the signal series corresponding to user PS1.
It is assumed that each frame includes the above-described unique word signal (reference signal) zone, and takes a configuration that allows error detection by a cyclic code (CRC: cyclic redundancy check).
In addition to the case where adaptive array processing is carried out at the base station to establish transmission or reception directivity, there are cases where adaptive array processing is carried out at the reception terminal side. A terminal that carries out adaptive array processing in such terminals (mobile station) is referred to as an “adaptive array terminal”.
Such an adaptive array terminal always carries out an adaptive array operation in both the reception mode and transmission mode. Therefore, the response vector of a signal from the terminal, when connected to the above-described SDMA base station, will vary for each frame, posing the problem that the communication quality may be degraded in multiple access.
This problem will be described in further detail hereinafter.
FIG. 22 schematically shows a state where radio communication is conducted between an adaptive array base station CS1 and respective terminals of an adaptive array terminal PS1 and a terminal PS2 that carries out the general non-directional transmission and reception.
Referring to FIG. 22, a plurality of the same signals arrive at SDMA base station CS1 by multipath propagation from adaptive array terminal PS1. The reception signal response vector of a signal from adaptive array terminal PS1 is represented as in the equations set forth below as a composite vector of a plurality of signals.X(t)=H11W1S1(t)+. . . +H1mW2S1(t)+H2S2(t)  (16)X(t)=H1S1(t)+H2S2(t)  (17)H1=H11W1+. . . +H1mW2  (18)
At SDMA base station CS1, the reception signal response vector (composite vector) for the signal from adaptive array terminal PS1 depends on the weights (W1, W2) of the transmission adaptive array processing of adaptive array terminal PS1.
This means that, when the weights of the transmission weight change at the adaptive array terminal PS1, the reception signal response vector will be altered at SDMA base station CS1 even if there is absolutely no variation in the propagation path itself.
In other words, the transmission weights may suddenly be shifted independent of variation in propagation due to terminal noise, calculation error, and the like.
When spatial multiplexing is to be carried out, SDMA base station CS1 measures the reception response vector for each of the multiple terminals. Multiple access communication (SDMA system communication) is allowed when the spatial correlation between the reception response vectors of the multiple terminals is equal to or below a threshold value.
Therefore, when adaptive array transmission is effected on the part of terminal PS1, there may be a case where the reception signal response vector viewed from the SDMA base station CS1 side varies suddenly and unpredictably, imposing the problem that the multiplex communication will be degraded in quality.
FIG. 23 shows a reception signal response vector H1 as a composite vector with respect to signals propagated via a plurality of paths from adaptive array terminal PS1.
Since such a reception signal response vector is altered depending on variation in the propagation path as described above as well as by other various factors, the reception signal response vector H1 applied as a composite vector may be altered more greatly than the variation in the propagation path.
The present invention is directed to solve the above-described problems. An object is to provide a radio terminal apparatus that conducts adaptive array processing, allowing radio communication with an SDMA base station while maintaining favorable communication quality, a transmission directivity control method and a transmission directivity control program thereof.